1. Field of the Invention
The present invention relates to a gripping assembly having reversible wedges, i.e. wedges having a self-unlocking ability, for gripping elongated objects such as cables or bars. More particularly, the invention relates to a gripping assembly comprising an elongated casing which is open at both ends and which defines a passage adapted for receiving said elongated object, and a pair of wedge-shaped clamping jaws, which are mounted in the passage of the casing for gripping said elongated object, said passage comprising two inner surfaces, which are disposed respectively on each side of the longitudinal axis of said passage and which converge towards one end thereof while making a predetermined angle with said longitudinal axis, each of said clamping jaws comprising an outer surface, which is in contact with one of the two inner surfaces of the passage of the casing, and an inner surface which faces said elongated object and in which a longitudinal groove is formed, the two clamping jaws being movable in a plane containing the longitudinal axis of the passage of the casing.
Reversible wedge gripping assemblies to which the present invention relates are used in numerous devices or apparatus, such as, for example, motor devices operating step by step, linear hydraulic winches, devices for anchoring cables or bars and certain handling devices. Generally, the reversible wedge gripping assemblies are used when it is desired to grip at least momentarily an object of elongated shape, for example for exerting a traction force on said object, and when the clamping jaws of the gripping assembly must be capable then of releasing the gripping pressure which they exert on said object, without disassembling the gripping assembly, by a simple relative movement in a direction opposite the pulling direction or by the action of a control device.
2. Description of the Prior Art
Numerous gripping assemblies have been proposed. They may be classified into two categories, namely gripping assemblies the clamping jaws of which have a slanting flat or conical outer surface, which is directly in contact with the inner flat or conical surface, slanted correspondingly, of the casing of the gripping assembly (see for example U.S. Pat. Nos. 1,637,270, 2,146,575, 2,958,916, 3,758,922 and 4,381,584, the patent DE No. 405 524 and the patent GB No. 13 230), and the gripping assemblies in which rollers or balls are interposed between the outer surface of the clamping jaws and the inner surface of the casing of the gripping assembly (see for example the U.S. Pat. Nos. 1,146,801 and 2,400,514 and the patent EP No. 0057622).
According to the general theory of wedges set forth more particularly in the publication "Techniques de l'Ingenieur", Mecanique et Chaleur, vol. 3, pages B 602-1 to 602-3, the driving-in force P, parallel to the axis of the wedge (clamping jaw) and the force Q normal to said axis, applied to the wedge, are related by the following relationship in the case of wedges with flat slanted surfaces: ##EQU1## in which .alpha. is the angle between the axis and a slanged face of the wedge (the axis being the straight line passing through the tip of the wedge and perpendicular to the base thereof), and .phi. is an angle such that tan .phi.=f, f being the coefficient of friction of the faces of the wedge (the outer face of a clamping jaw and the inner face of the casing of the gripping assembly) which are mutually in contact. In the case of wedges with conical surface, the formula (1) becomes: ##EQU2## .alpha. being in this case the angle between the axis and a generating line of the conical surface of the wedge.
In both cases, the condition of reversibility of the wedge is .alpha.&gt;.phi..
When the object to be gripped is a cable or a bar with substantially round cross section, the inner surface of the clamping jaws of the gripping assembly, i.e. the surface of the clamping jaws which is in contact with the object to be gripped, usually comprises a groove with semi-circular cross section, the radius of which corresponds to the radius of the cross section of the cable or the bar to be gripped. In this case, assuming that the pressure due to gripping is transmitted hydrostatically to the inside of the cable, the force of adhesion or friction force F between each clamping jaw of the gripping assembly and the cable is given by the formula: EQU F=.pi..multidot.Q.multidot.f' (3)
in which Q has the meaning already given and f' represents the coefficient of friction between the clamping jaw and the cable. Replacing Q by its value, given by formula (1) or by formula (2) we obtain: ##EQU3##
Formulas (4) and (5) may further be written: EQU F=kP (6)
with: ##EQU4## in the case of clamping jaws with flat outer surface, and: ##EQU5## in the case of clamping jaws with conical outer surface.
So that there is no sliding between each clamping jaw and the cable to be gripped, the absolute value of the friction force F must be higher than or equal to the absolute value of the driving-in force P (these two forces are oriented in opposite directions). In other words, k must be greater than or equal to 1. To this end, according to formulas (7) and (8), it can be seen that f' must be as large as possible and/or that (.alpha.+.phi.) must be the smallest possible.
For example, with a steel cable and steel clamping jaws having a groove with a smooth surface, f' is equal to 0.1. For steel clamping jaws, the outer surface of which is directly in contact with the inner surface of the casing of the gripping assembly, the angle .phi. is about equal to 6.degree.. Taking into account the condition of reversibility (.alpha.&gt;.phi.) the angle .alpha. is chosen for example equal to 8.degree.30'. Thus, according to formulas (7) and (8), k is equal to 0.61 or 0.39 depending on whether the clamping jaws have a flat or conical outer surface. In both cases, it can be seen that the friction force F is smaller than the driving-in force P and therefore sliding occurs between the clamping jaws and the cable.
To avoid any sliding between the clamping jaws and the cable, a first known solution consists in greatly increasing the coefficient of friction f' by scoring or notching the surface of the groove of the clamping jaws which is in contact with the cable (see for example the U.S. Pat. Nos. 1,637,270, 2,146,575, 2,958,916, 3,758,922 and 4,381,584). However, the drawback of scores or notches is that they damage the outer surface of the cable, in particular when a high gripping pressure must be applied to the cable, for example for exerting a heavy traction force thereon.
Another known solution consists in considerably reducing, even cancelling out the value of the angle .phi., i.e. in considerably reducing or suppressing the friction between the outer surface of the clamping jaws and the inner surface of the casing of the gripping assembly by interposing rollers or balls between these surfaces, as shown for example in the U.S. Pat. Nos. 1,146,801 and 2,400,514 or in the patent EP No. 0057622. This further allows to reduce the value of the angle .phi. while still satisfying the condition of reversibility. For example, with rollers (.phi..congruent.0) and with clamping jaws having a slope of 0.1 (.alpha.=5.degree.42'), and with a coefficient of friction f'=0.1, according to formula (7) k is equal to 1.57. In this case, the friction force F between the cable and the clamping jaws is much greater than the driving-in force P and no sliding occurs between the clamping jaws and the cable. However, for equal gripping forces and equal traction forces, the gripping assemblies with rollers or balls have the drawback of being of a size, weight and cost higher than the gripping assemblies in which the clamping jaws are directly in contact with the inner surface of the casing of the gripping assembly. Furthermore, since the contact pressures between the rollers, on the one hand, and the outer surface of the clamping jaws and the inner surface of the casing of the gripping assembly on the other hand, are locally greater than in the case where the outer surface of the clamping jaws is directly in contact with the inner surface of the casing, the clamping jaws and the casing must be made from very hard steels or their surfaces in contact with the rollers must be treated, for example by a heat treatment, so as to make them hard and capable of withstanding the locally higher contact pressures, which further increases the cost of gripping assemblies having rollers.